Processing and Application of Ceramics 11 [2] (2017) 154–159

https://doi.org/10.2298/PAC1702154C

Microwave assisted sol-gel synthesis of high dielectric constant CCTO

and BFN ceramics for MLC applications

Sonia, Mallam Chandrasekhar, Pawan Kumar∗

Department of Physics, National Institute of Technology, Rourkela, Odisha 769008, India

Received 12 December 2017; Received in revised form 15 April 2017; Received in revised form 23 May 2017; Accepted

5 June 2017

Abstract

Ba(Fe1/2Nb1/2)O3 (BFN) and CaCu3Ti4O12 (CCTO) ceramic powders were synthesized by microwave assistedsol-gel synthesis technique and sintered at 1100 °C and 1000 °C, respectively. Calcination and sintering pro-cesses were carried out in a microwave furnace. Dielectric constant (εr) ∼2450 and dielectric loss (tan δ) ∼0.5at frequency of 1 kHz and 20 °C were observed for the BFN ceramic samples. Higher value of εr ∼ 3600 andlower value of tan δ ∼ 0.07 at frequency of 1 kHz and in 20–60 °C temperature range for the CCTO ceramicsamples suggested its utility for MLC applications. Sharp decrease of εr and sharp increase of tan δ at higherfrequencies of BFN ceramic samples indicated the presence of Debye like relaxation.

Keywords: Ba(Fe1/2

Nb1/2

)O3, CaCu

3Ti

4O

12, microwave assisted sol-gel process, dielectric properties

I. Introduction

With the increase in miniaturization scaling of elec-

tronic devices, requirements of high dielectric con-

stant (εr) materials with good temperature and fre-

quency stability are increasing [1]. Generally, high ca-

pacitance in a material is related to its ferroelectric na-

ture [2]. Recently, high dielectric response has been ob-

served in non-ferroelectric materials. Ba(Fe1/2Nb1/2)O3

(BFN) and CaCu3Ti4O12 (CCTO) ceramics are the non-

ferroelectric systems showing excellent dielectric prop-

erties [3–5]. Wang et. al. [4] reported dielectric char-

acteristics of BFN ceramics over a broad temperature

and frequency range. In BFN system, giant dielectric

behaviour with very strong frequency dispersion was

observed in 406–650 K temperature range [4]. CCTO

ceramics has also attracted much interest due to its ex-

traordinary dielectric properties [5]. This system also

exhibits giant dielectric permittivity, which is almost

constant in 100–400 K temperature range [6]. In BFN

and CCTO ceramics, neither a phase transition nor a

crystal structure change is detected [7]. Therefore, high

dielectric constant in BFN and CCTO systems is con-

sidered to be extrinsic in nature. In spite of seemingly

important possible dielectric applications, dielectric loss

∗Corresponding author: tel/fax: +91 661 2462726,

e-mail: pvn77@rediffmail.com, pawankumar@nitrkl.ac.in

(tan δ) of these ceramics is very high [8]. The major

drawback of BFN and CCTO ceramics is the difficulty

of obtaining high density and single phase formation by

conventional solid state synthesis route [9]. The solid

state route (SSR) requires high processing temperatures,

which is detrimental to multilayer ceramic capacitors

(MLCC) because of the electrode oxidation. On the

other hand, chemical technique, often referred to as sol-

gel processing, has advantage of control over purity and

stoichiometry with the reduction of processing tempera-

tures [10]. Materials produced with this technique have

lower calcination and sintering temperatures with grains

of submicron size, which allow thinner layers and en-

hanced dielectric breakdown strength with the lower di-

electric loss. Although, processing temperatures are de-

creased in sol-gel processing technique, the processing

duration is nearly still the same as that of the solid state

reaction route. With a longer processing duration, the

chances of creation of non-uniform grains, defects and

development of pores are high. These disadvantages of

the sol-gel processing technique can be avoided by us-

ing microwave processing technique. The microwave

processing technique has several advantages like: rapid

heating, penetrating radiation, obtaining more uniform

microstructure and hence higher density. Moreover, this

technique is inherently dry and fumeless process, which

means environmentally clean, therefore it is also known

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Sonia et al. / Processing and Application of Ceramics 11 [2] (2017) 154–159

as clean processing technique [11,12]. In the microwave

processing of ceramics, electromagnetic waves interact

with the ceramic materials, leading to volumetric heat-

ing. The presence of high dielectric loss in BFN and

CCTO ceramic samples can be exploited for their syn-

thesis with lower processing temperature and time by

using microwave synthesis technique. Microwave heat-

ing also has the potential for energy and cost savings

when compared with conventional heating processes.

Besides that, nearly theoretical density and uniform

grains, which in turn improves physical properties of

ceramics, can be achieved in microwave processed ce-

ramic samples [11,12]. Therefore, the synthesis of BFN

and CCTO ceramic samples by microwave assisted sol-

gel processing technique can help obtaining dense and

uniform grain size with pore free microstructures, which

can improve the material’s electrical properties.

In the present work, dielectric properties of BFN and

CCTO ceramic samples, synthesized by microwave as-

sisted sol-gel process are reported and discussed in de-

tail.

II. Experimental

Ba(Fe1/2Nb1/2)O3 (BFN) and CaCu3Ti4O12 (CCTO)

ceramic samples were synthesized using grade reagents

barium acetate, calcium acetate, niobium chloride, fer-

ric nitrate, copper acetate and titanium isopropoxide as

starting precursors. For the synthesis of BFN ceramic

powders, stoichiometric amount of barium acetate and

niobium chloride were separately dissolved in acetic

acid and concentrated HCl solvents, respectively, with

continuous magnetic stirring. Stoichiometric amount of

ferric nitrate was dissolved in distilled water. These so-

lutions were mixed together and stirred slowly with

continued heating until a transparent solution was ob-

tained. For the synthesis of CCTO ceramic powders,

stoichiometric amounts of calcium acetate and copper

acetate were dissolved together in acetic acid with con-

tinuous magnetic stirring at 70 °C. Once calcium ac-

etate and copper acetate were dissolved, stoichiometric

amount of titanium isopropoxide was added to the so-

lution and stirred slowly with continued heating until a

transparent solution was obtained. Transparent solutions

of BFN and CCTO systems were heated at ∼110 °C to

form precipitates and finally dried powders. To deter-

mine the sintering temperatures, the dried powders were

subjected to DSC/TGA characterization by using a ther-

mal analyser (Netzsch, Germany STA449C/4/MFC/G).

Calcinations of BFN and CCTO dried powders were

carried out in a microwave furnace at 900 °C for 40

minutes and at 800 °C for 40 minutes, respectively.

Development of phase formation was studied by us-

ing X-ray diffraction (XRD) (Rigaku Ultima IV X-ray

diffractometer, Tokyo, Japan) technique. Calcined pow-

ders were mixed thoroughly with 2 wt.% polyvinyl alco-

hol (PVA) and pressed into disks of diameter ∼10 mm

and a thickness ∼1.5 mm under ∼60 MPa pressure us-

ing a hydraulic press. Microwave sintering of BFN and

CCTO pellets was carried out at 1100 °C for 1 h and

1000 °C for 1 h, respectively, with a heating rate of

40 °C/min by placing the green pellets at the centre of

4.4 kW, 2.45 GHz multi-mode microwave cavity [12].

Microwave furnace temperature was recorded by us-

ing a Raytek non-contact sensor (XRTG5). Experimen-

tal density of the sintered samples was calculated by

using Archimedes method. Microstructures of the sin-

tered samples were studied using JEOL T-330 scanning

electron microscope (SEM). Silver paste was applied

on both sides of sintered pellets and fired at 400 °C for

30 min for good adhesion. Dielectric properties (εr and

tan δ) were measured as a function of temperature at

different frequencies using computer interfaced HIOKI

3532-50 LCR-HITESTER.

III. Results and discussion

Figure 1a shows the DSC and TGA curves of the un-

calcined BFN powder. From room temperature (RT) to

1200 °C, the overall weight loss is ∼35%. In the un-

calcined BFN powder, reactions between the starting

precursors are starting at ∼300 °C and completing at

∼900 °C. Figure 1b shows the DSC and TGA curves

of the uncalcined CCTO powder. There is also an over-

all weight loss of ∼45% from RT to 30 °C, which can

be attributed to the loss of solvents and non-structural

Figure 1. DSC and TGA curves of the uncalcined: a) BFN and b) CCTO powders

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Figure 2. XRD patterns of the sintered: a) BFN and b) CCTO ceramic samples

water desorption and destruction of xerogel network. In

the DSC plot, exothermal peaks at ∼300 °C can be at-

tributed to the combustion reactions, and formation of

intermediate phases. DSC peak at ∼1000 °C can be at-

tributed to the decomposition and reactions between the

intermediate phases to form the CCTO compound.

Figure 2 shows XRD patterns of the sintered BFN

and CCTO samples at 1100 °C and 1000 °C for 1 h each,

respectively. The presence of single perovskite phase

peaks can be seen from XRD pattern of the sintered

BFN sample, while small amount of secondary phase

is observed in XRD pattern of the sintered CCTO ce-

ramics. Secondary phase fraction (SPF) in the sintered

CCTO sample is calculated by using the following equa-

tion [13]:

SPF =Is

Ip + Is

(1)

where, Ip and Is are intensities of the most intense peaks

corresponding to perovskite and secondary phases, re-

spectively. The secondary phase is found to be CuO,

which is below 5% in the sintered CCTO ceramic sam-

ple. Compared to other processing techniques [14–16],

the microwave assisted sol-gel processing has signif-

icantly reduced the processing temperature and time,

which highlights its advantage.

Figure 3 shows SEM micrographs of the sintered

BFN and CCTO ceramics. Dense distribution of grains

can be observed in both ceramic samples. Grains with

size ranging from ∼2 µm to 10 µm and from 1 µm to

4.5 µm were observed in the sintered BFN and CCTO

ceramic samples, respectively. In comparison to the

BFN sample, a slight decrease of average grain size is

observed in the CCTO ceramics. The average grain size,

calculated by using linear intercept method, is found to

be ∼3 µm and 1 µm for the BFN and CCTO ceramic

samples, respectively. Intatha et al. [14] and Eitssayeam

et al. [15] reported grain size of ∼9 µm in convention-

ally prepared BFN ceramics. Zang et al. [16,17] synthe-

sized CCTO samples through conventional solid state

reaction route and reported grain size of ∼10 µm. The

present study highlights that through microwave pro-

cessing technique we can obtain fine and uniform dis-

tribution of grains with dense morphology.

Experimental density of the sintered BFN and CCTO

samples (dexp) is calculated by using Archimedes’ prin-

ciple [18]:

dexp = dk

wdry

wsoa − wsus

(2)

where wdry, wsoa, and wsus are the dry weight, soaked

weight and suspended weight of the pellet, respectively,

Figure 3. SEM micrographs of the sintered: a) BFN and b) CCTO ceramic samples

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Sonia et al. / Processing and Application of Ceramics 11 [2] (2017) 154–159

Figure 4. Frequency dependence of εr and tan δ of: a) BFN and b) CCTO ceramic samples

and dk is the density of the kerosene oil. The ratio of the

experimental density to the theoretical density (TD) is

known as the relative density [19]. Experimental densi-

ties of the BFN and CCTO ceramic samples are ∼6.28

and 4.73 g/cm3, which corresponds to ∼96 %TD and

95 %TD, respectively. Experimental density of the mi-

crowave processed BFN and CCTO ceramic samples is

nearly equal to the same samples processed convention-

ally [14–16].

Figure 4 shows the frequency dependence of εr and

tan δ of the sintered BFN and CCTO ceramics. Fre-

quency dependence dielectric spectra of both samples

reveal that εr decreases and tan δ increases with the in-

crease of frequency. The net polarization of a material

is the sum of ionic, dipolar, electronic and interfacial

polarizations. At low frequencies, all the polarization

mechanisms contribute to εr whereas different polariza-

tion mechanisms filter out with increasing frequency.

Therefore, εr contributions of both samples decrease

with the increase of frequency, which results in decrease

of dielectric constant [12,20]. In order to clearly distin-

guish between the effects of different polarizations on

dielectric properties broadband dielectric study must be

performed. All the polarizations are active in the low

frequency region of applied external AC electric field,

and with the increase of frequency the relaxation pro-

cesses sets in the materials, which results in the high di-

electric loss. At lower frequencies, all the dipoles easily

respond to the applied frequency whereas with increas-

ing frequency, the dipoles cannot orient instantaneously

to the frequency leading to the lag in the polarization

and dielectric loss. Therefore, tan δ of both samples in-

creases with the increase of frequency [21]. High dielec-

tric loss at low frequencies for the BFN ceramic sam-

ple suggests the presence of relaxation frequency in the

lower frequency range. In MHz frequency region, sharp

decrease in εr and increase in tan δ is observed in the

BFN ceramic sample, which indicates Debye like relax-

ation [22].

Figures 5a,c show the temperature dependence of the

dielectric constant (at different frequencies) for the sin-

tered BFN and CCTO ceramics. Dielectric values for

both ceramic samples at selected temperatures and fre-

quencies are given in Table 1. Temperature coefficient of

capacitance (Tcc) is a very important parameter for de-

ciding about the utility of a particular dielectric material

for MLC applications. Tcc, within a temperature range

of TI to TII , is defined with the following equation:

Tcc =εr(TII) − εr(TI)

εr(TI)(3)

where εr(T ) is dielectric constant at temperature T . In

the 20–60 °C temperature range, value of Tcc is negli-

gible for both BFN and CCTO ceramic samples. Fig-

ures 5b,d show the temperature variation of tan δ at dif-

ferent frequencies for the sintered BFN and CCTO ce-

ramics. It can be seen that the BFN ceramics has higher

tan δ than the CCTO ceramics. Dielectric loss at 1 MHz

frequency rises suddenly in both ceramic samples. At

lower frequency, different polarizations, present in the

materials can follow the field variations. However, with

the increase of frequency, polarizations start lagging be-

hind and account the rise in dielectric loss. Higher value

of εr ∼ 3600 and lower value of tan δ ∼ 0.07 at fre-

quency of 1 kHz for the CCTO ceramic sample in 20–

60 °C temperature range hint about its utility for MLC

applications.

Dielectric properties of the obtained BFN ceramics

are comparable with the earlier reports [14,23–25]. In

the present study, εr is lower than in the earlier reports

[14,23–25], but low Tcc, low tan δ in 20–60 °C temper-

ature range of the BFN ceramics, processed at lower

processing temperature and duration, signifies the im-

Table 1. Dielectric properties of BFN and CCTO ceramic samples at 1 kHz

SampleCalcination Sintering Grain size εr εr tan δ tan δ

temperature [°C] temperature [°C] [µm] at 20 °C at 60 °C at 20 °C at 60 °C

BFN 900 1100 4 2450 2500 0.5 0.5

CCTO 800 1000 1.5 3600 3600 0.075 0.070

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Figure 5. Temperature dependence of: a) εr, b) tan δ for BFN, and c) εr, d) tan δ for CCTO ceramic samples

portance of present synthesis route. Dielectric loss of

the microwave assisted sol-gel synthesized and sintered

CCTO ceramics is much lower than that of earlier re-

ports [26]. This suggests that the microwave assisted

sol-gel synthesis technique is helpful to inhibit the grain

growth and lowers the dielectric loss of CCTO ceramic

samples [27].

IV. Conclusions

Ba(Fe1/2Nb1/2)O3 (BFN) and CaCu3Ti4O12 (CCTO)

ceramic powders were synthesized by microwave as-

sisted sol-gel technique and sintered at relatively low

processing temperatures and durations. Dielectric prop-

erties of the sintered CCTO ceramic samples are bet-

ter than for BFN ceramic samples. Higher value of

dielectric constant, lower value of dielectric loss with

negligible temperature coefficient of capacitance of the

prepared CCTO ceramics suggested its usefulness for

MLC applications. Lower processing temperature and

processing durations highlighted the advantage of mi-

crowave assisted sol-gel synthesis technique.

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